1. Technical Field of the Invention
This invention relates to the control and reduction of fluid pressure in control valves, especially but not exclusively severe service valves for use in power industries and oil and gas wells.
2. Description of the Related Art
The technology most widely used in severe service valves utilizes pressure letdown chambers consisting of one or more flow passages containing multiple orifice openings, labyrinths, or multiple, abrupt angular turn, passageways resulting in a staged pressure reduction. Alternatively, flow restrictions can be afforded by providing in the flow passage a region of reduced cross sectional area through which the fluid passes. As the fluid flows through the flow passage, the velocity is locally increased in that region thereby generating turbulence which dissipates energy and reduces the pressure.
In the afore-mentioned control valves, dissipation of the energy of the flowing fluid is effected by frictional drag through tortuous passages or by successive abrupt contractions and expansions through smooth passages. Both of these types of valve work well for clean fluids but in a number of applications the fluid will contain contaminants, for example solid particles or droplets of liquid, which will cause the passages to erode quickly. This is especially prevalent in the tortuous passage type which promotes the collision of particles with the passage walls, thereby accelerating erosion, and in expansion/contraction valves as the contraction has an associated acceleration and the energy dissipated through the physical flow restrictions can result in physical damage or erosion to valve components if not controlled in a careful manner. The problem of erosion is particularly prevalent in well head choke valves, which are the primary shut-off/control valve on the top of an oil or gas well head. As this valve is immediately atop the well head there is no possibility of removing contaminants prior to them entering the valve.
A solution to the aforementioned problem is disclosed in our co-pending UK Patent Application number 0618165. While this is a very effective solution, the cost of ceramics needed in higher erosion solutions can be substantial, hence it is advantageous to effect the maximum pressure reduction in the most compact configuration possible, thus the higher fluid resistance a valve trim has the better.
The present invention attempts to mitigate problems with existing valves by providing a compact control valve with increased erosion resistance.